A graphite is generally used as a negative electrode material for lithium ion battery. Theoretical capacity of the graphite is 372 mAh/g. Theoretical capacity of Si and Sn is higher than that of the graphite, and therefore if Si or Sn can be used as the negative electrode material, a lithium ion battery having high capacity can be provided. However, particles containing Si or Sn originally have low electric conductivity and also a volume change associated with intercalation and deintercalation of lithium ions is large, and therefore the particles are broken and an electrically conducting path comes to a disconnection, leading to an increase in internal resistance. Thus, various proposals have been made on composite materials composed of a carbon material and Si particles complexed with the carbon material.
For example, Patent Literature 1 proposes a negative electrode material for lithium secondary battery, in which the negative electrode material is composed of silicon particles and a carbon precursor, a coating layer composed of a non-graphitic carbon material is formed on a surface of the silicon particles, and a peak intensity ratio of silicon to carbon in X-ray photoelectron spectroscopy (ESCA) is 0 to 0.2.
Patent Literature 2 proposes a negative electrode material for lithium ion secondary battery, in which the negative electrode material comprises particles composed of silicon and carbon-coating layer on a surface of the particles, an average particle size of core particles is not less than 5 nm and not more than 100 nm, and an area of a peak near 104 eV derived from silicon oxide is smaller than 25% of a sum of areas of peaks near 100 eV derived from silicon and silicon-carbon in X-ray photoelectron spectroscopy.
Patent Literature 3 discloses a negative electrode material for lithium secondary battery, in which the negative electrode material comprises Si particles having an average particle size of 0.05 to 5 μm and several types of carbonaceous material, and an oxygen content of not more than 5% by weight.
Patent Literature 4 proposes a composite electrode material, in which the composite electrode material comprises graphite particles, amorphous carbon and silicon, and has a silicon content in terms of SiO2 of 40 to 80% by weight, true density of 1.8×103 kg/m3 or more, tap density of 0.8×103 kg/m3 or more, and a specific surface area of 8×103 m2/kg or less, and has one peak near 102.5 to 107.5 eV in a Si2P spectrum in XPS.